Linux 2.6.15 has been released after two months and a week of development. You can check the comprehensible changelog or grep the full changelog. There are some new features like shared subtrees, UDP fragmentation offload, PPP MPPE encryption (VPN), NTFS write support (except for creating files), PPC64 thermal improvements, support for the late-2005 powerbook series, SATA passthru support (for SMART), console rotation for fbcon, nf_conntrack subsystem, some scalability and performance improvements, and lots of other changes. As always, download it from Kernel.org, or wait for your distributor to ship it.